9 research outputs found
Origin and evolution of ferroelectricity in the layered rare-earth-titanate, TiO, Lichtenberg phases
We report a systematic first-principles study based on density functional
theory (DFT) of the structural and ferroelectric properties of the
TiO perovskite-related oxides with La, Ce, Pr, and Nd. We
show that, in all cases, the ferroelectric distortion to the ground-state polar
structure from its parent centrosymmetric phase is driven by a
single polar soft mode consisting of rotations and tilts of the TiO
octahedra combined with displacements of the ions. A secondary
centrosymmetric distortion, which is stable in the parent structure,
contributes substantially to the energy lowering of the ground state. We
evaluate the trends in structure and polarization across the series as a
function of and reconcile discrepancies in reported values of polarization
in the literature. Our results confirm that the family of TiO
materials belong to the class of proper geometric ferroelectrics.Comment: 15 pages, 12 figures, 7 table
Elasticity of Diamond at High Pressures and Temperatures
We combine density functional theory within the local density approximation,
the quasiharmonic approximation, and vibrational density of states to calculate
single crystal elastic constants, and bulk and shear moduli of diamond at
simultaneous high pressures and temperatures in the ranges of 0-500 GPa and
0-4800 K. Comparison with experimental values at ambient pressure and high
temperature shows an excellent agreement for the first time with our
first-principles results validating our method. We show that the anisotropy
factor of diamond increases to 40% at high pressures and becomes temperature
independent.Comment: 10 pages, 3 figures, 1 tabl
Thermoelastic Properties of Ringwoodite [Fe_x,Mg_(1-x)]_2SiO_4: Its Relationship to the 520 km Seismic Discontinuity
We combine density functional theory (DFT) within the local density
approximation (LDA), the quasiharmonic approximation (QHA), and a model
vibrational density of states (VDoS) to calculate elastic moduli and sound
velocities of gamma-[Fe_x,Mg_(1-x)]_2SiO_4 (ringwoodite), the most abundant
mineral of the lower Earth's transition zone (TZ). Comparison with experimental
values at room-temperature and high pressure or ambient-pressure and high
temperature shows good agreement with our first-principles findings. Then, we
investigate the contrasts associated with the
beta-to-gamma-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and
temperatures relevant to the TZ. This information offers clearly defined
reference values to advance the understanding of the nature of the 520 km
seismic discontinuity.Comment: 29 pages, 6 figures, 2 tables. Under Revie
Evidence for a pressure-induced spin transition in olivine-type triphylite
We present a combination of first-principles and experimental results regarding the structural and magnetic properties of olivine-type under pressure. Our investigations indicate that the starting phase of persists up to 70 GPa. Further compression leads to an isostructural transition in the pressure range of 70â75 GPa, inconsistent with a former theoretical study. Considering our first-principles prediction for a high-spin to low-spin transition of close to 72 GPa, we attribute the experimentally observed isostructural transition to a change in the spin state of in . Compared to relevant Fe-bearing minerals, exhibits the largest onset pressure for a pressure-induced spin state transition